by Greg Marley
Undoubtedly, it was by observing pigs and other larger mammals unearth and eat the fine European truffles that rural people began to use pigs as guides to find and expose them. One problem with using pigs as truffle hunters is that the great ravenous rooters love to find truffles for their own dining pleasure, so truffle hunters have to be quick to ensure their prize ends up in the basket and not down the pig’s throat. In the past, it was not unusual to see truffle hunters with mangled or missing fingers, the legacy of wresting truffles from the jaws of a hungry mushroom-loving pig. Today, truffle hounds have largely replaced pigs. They tend to be better companions, can ride in the front seat of the truck, and, best of all, are happy to get a dog treat as a reward rather than consume the treasure they locate and unearth. Most important is that a dog’s olfactory ability rivals that of the snuffle pig.
It’s an understatement to say that edible truffles are highly prized. There is an intense passion and mystique reserved for the best Italian and French truffles that rivals the feeling for any other food. In 2007, a new record price was set at auction for an Italian white truffle from the Piedmont town of Alba. A group of Hong Kong enthusiasts, with very deep pockets, paid 210,000 US dollars for a truffle weighing 750 grams. That was about $127,000.00 per pound! Alba, Italy, and her famous truffles were again in the news in February 2009 when an unnamed businessman and his five guests sat down to a dinner at Cracco’s, one of the world’s top restaurants, and, without looking at the menu, ordered white truffles. When the waiter presented the businessman with a $5,058.00 bill, he balked and protested, claiming he wasn’t told the cost or the weight of the fungi, but finally agreed to pay half. At last report, the matter was headed for a court resolution.
Truffle Evolution
Truffles have evolved in many regions of the world from a diversity of ancestors over geologic time. Karen Hansen, a research associate with Harvard’s Farlow Herbarium of Botany, has done extensive molecular and genetic examinations of the Ascomycete truffles and estimates that the truffle lifestyle has evolved independently at least fifteen times within six different families in the order Pezizales alone.8 Many epigeous members of the cup fungi form deep urn-like cups at or just below the soil surface. Others have cups almost completely enclosed with only a small opening at the apex. It involves rather small and incremental steps to form fruit remaining underground and spore sacs that no longer forcibly eject their cargo into the air. Examples exist of species in all phases of the evolutionary progression from open cup to enclosed and then to more complex and convoluted structure. Some, like most members of the genus Tuber, are compact, spherical, and dense with a network of light colored veins running through the spore-bearing gleba. Others are more simple, folded cups, with hollow spaces between but no opening.
Several morphological steps must occur in order for a fungus to be considered evolved into truffledom:
• The spore-bearing tissue must become enclosed within a skin that will protect the spores while they mature. Many of the non-Ascomycete truffle-like species have evolved from genera that have a well-developed annulus or partial veil that at times persists into maturity, covering the gills.
• The spore-release mechanism loses the ability for explosive or forcible discharge.
• The mature fruiting body develops a distinctive and strong odor, signaling to animal mycophagists that dinner is ready. The animals become the mechanism to get the spores to the surface of the ground for release into the environment.
• This last point is somewhat conjectural on my part. We know that essentially all truffle-like fungi form mycorrhizal associations with woody plants. The nature of the symbiotic relationship generally ensures that the mycelial colony of the fungus is perennial, existing for a number of years associated with the same host tree. It could be that the perennial nature of the vegetative component of the fungi confers a stability that, early in the evolution toward hypogeous status, allows for greater latitude in fruiting failure while still ensuring survival of the individual over time. If, while developing sufficient scent to attract foraging animals, there are years in which no spores make it to the soil surface, the stability of the mycelium helps to ensure survival of the fungus. An organism with a less stable life-course would have less chance of survival. As I said—conjecture.
The evolutionary pathway to a hypogeous lifestyle must be effective since it’s happened repeatedly in numerous fungal groups on several continents. Australia, which may represent the nirvana of truffle evolution, has the highest number of hypogeous fungi when measured as a percentage of the overall fungal population. This is the case, even though it is acknowledged that Australian fungi—particularly the truffles—have not been well studied. The evolution toward a hypogeal habit is reported to occur more frequently in warmer and drier climates since fungi have a harder time protecting the fragile spore-making tissue from drying. The protection afforded by underground development is a major boost in likely success.9
Truffle Ecology: The Pivotal Role of Mycorrhizal Fungi
Truffles are most abundant in the first few inches of soil beneath trees and other woody plants. That organic layer of soil is the most biologically active; it is where dead leaves, needles, twigs, and other organic matter are broken down and recycled and the nutrients that are bound up in their tissue are released. It is estimated that a single teaspoon of healthy forest soil might contain as much as 100 meters of fungal mycelium and that with each step we take, our feet cover several miles of fungal strands busily invigorating the forest.
We know that the mutualistic fungi-plant associations we call mycorrhizal likely began shortly after plants and fungi emerged from the primordial seas and colonized the land. Though fossil records are somewhat scant due to the delicate nature of plant tissues, we have evidence that indicates club mosses formed primitive fungus-root structures as early as 400 million years ago. Today, essentially all gymnosperms and 80 percent of angiosperm plants form mycorrhizal associations with fungi. A number of plants can live independent of fungal associations and have evolved to be successful colonizers in new territory and a number of the most invasive weeds in the world fit into this group.10 But most plants are mycorrhizal-dependent, meaning that they readily accept at least one fungal mycobiont into their tissues and that their long-term survival is dependent on the formation of these symbiotic relationships. Often these plants can function without a fungal association for brief periods of time, especially in nutrient-rich soil, but they appear nutrient deprived, stunted, and sickly.
All of the known truffle-producing fungi form mycorrhizal associations with woody plants, mostly trees and shrubs. Mycorrhizal symbionts are essential components of a healthy forest and their perpetuation is necessary for the survival of the forest. Sometimes foresters learn the hard way what an essential role mycorrhizal fungi play in the survival of trees, as happened with an outplanting of Douglas fir seedlings in a nursery field in Oregon in the 1960s.11 The field, which was converted from potato cultivation, was fumigated with a strong fungicide prior to planting the tree stock because of concern about lingering fungal diseases. Because the fumigation eliminated residual soil mycorrhizal fungi for the seedlings, the firs quickly became stunted and sickly and had a high mortality rate the first year and an elevated rate the second year despite the application of fertilizer and adequate irrigation. There were, however, islands of thriving seedlings where wind-borne or residual soil spores established mycorrhizal associations with the emerging seedlings. These islands of normal vigorous growth spread out as the fungal mycelium expanded.
All fungi are examples of “more than meets the eye,” but none more so than truffles and their cornerstone relationships with the trees and animals they nourish. A significant percent of the fungal symbionts of forest trees produce underground fruiting bodies. The perpetuation of these mushroom species is vital for the ongoing health of the forests, and their perpetuation depends on the vitality of the population of animals who locate, unearth, and consume these
truffles. The next time you have the opportunity to take a nighttime forest stroll, listen for the sounds of the flying squirrels as you swat the odd mosquito. The squirrels have a characteristic behavior as they collect and store nuts or fungi. They place the food in a shallow cavity or into the V formed by two intersecting branches and then rear back and jam the food item into place by vigorously hammering with their forepaws. It makes a distinct “thwak thwak thwak” sound. Remind yourself that the health of the forest might depend on the success of this seldom-seen nocturnal squirrel and its relationship with the rarely seen forest truffles.
18
WOODPECKERS, WOOD DECAY FUNGI,
AND FOREST HEALTH
On meadows, where were wont to camp
White mushrooms, rosy gilled,
At dawn we gathered, dewy-damp,
Until the basket filled!
ANON, REMINISCENCES OF CHILDHOOD
FROM SONGS OF LUCILLA, 1901
Henry has been an avid duck hunter since he was a young teen and, as a man, he married the right woman with all the skills needed to make duck and sauerkraut, his favorite dish. Today, decades later, he walks quietly through prime wood duck nesting habitat with not a bird in sight. The mature hemlocks overhang the still river water on this late spring day. As he seeks his quarry, his favorite shotgun lies cradled in his arms, specially hand-loaded cartridges in both barrels. He has gone light on the powder, but packed in with the wadding is a special ingredient that makes this hunt unique. Carefully inserted into the hollow shotgun slug is a softwood dowel colonized with the cultivated mycelium of the red-belted polypore, Fomitopsis pinicola. There across the river is his prey, partially hidden in the gloom of the dense overstory but exposed by the opening over the river. From seventy-five feet he takes careful aim and lets fly. When the smoke clears he can clearly see the gash on the trunk of the large hemlock where his payload has shredded bark, imbedding slugs into the pale softwood cambium some thirty feet above the river surface. His hope and plan is that by forcibly inoculating the tree with the vegetative “seed” of the fungus, sometime in the next five to fifteen years, his small investment in time and shotgun ammo will turn into a decaying mature hemlock sprouting red-belted conks and containing several new woodpecker cavities and their associated tenants. On this particular day he is not shooting to kill a duck, but aiming to create habitat for future generations of woodpeckers, owls, flying squirrels, cavity-nesting ducks, and their kin. But this story starts in another place in time and space.
The Changing Forest Landscape
We have changed the face of this planet through the fruits of our labor, our burgeoning numbers, and our need for homes, food, and stuff: lots of stuff. From almost the first moment Europeans landed on the shores of the Americas, settlers began harvesting the seemingly endless forest that marched inland from every shore. Trees provided fuel for our fires and timber for ships, homes, and towns. They also posed an almost impenetrable obstacle to farming, grazing livestock, and westward expansion in the early days of European colonization. For the next 300 years, as settlers explored, conquered, colonized, and otherwise domesticated much of this great land, the forests became less like a wilderness and more like a renewable resource to be cut, grown, and cut again. Most forested regions of America have been through a number of tree harvesting cycles. The virgin forests are but a dream and the remaining timber is younger—a second, third, or fourth growth following successive clearing operations. Forest management has become a science and a business, designed to maximize marketable timber harvesting and to reduce the time required to mature a generation of trees to marketable size. Over the past century, this timber management strategy led to an increasingly narrow mix of tree species and a movement toward stands of trees all the same age and size. We have actively, even aggressively, managed many forests for the production of softwood conifer species, the most valuable for lumber or pulp production—at the expense of deciduous trees like oaks, beech, and other nut trees. In recent years, foresters and ecologists have found that a decreasing diversity of tree age and species brings with it a decreased diversity and populations of many animals that rely on a mix of tree species and the presence of large snags and mature old trees.
A snag is a dead standing tree, generally defined as at least 8 inches in diameter (though often much larger) and of sufficient height to stand well above the forest floor. Large snags and living trees of large girth now are recognized as a vital part of a healthy forest community because they provide food, shelter, and nesting sites for the birds and mammals that have evolved to live in tree cavities. These include birds that are able to create their own cavities (known as primary cavity nesters or PCNs) such as large woodpeckers and flickers. They also encompass other birds, mammals, and insects that move into abandoned cavities made by woodpeckers and naturally occurring cavities (called secondary cavity nesters, SCNs). One other group of important cavity creators or cavity preparers is the rarely acknowledged wood-degrading, heart-rot fungi.
The Role of Heart-Rot Fungi
Heart-rotters are a group of fungi that specialize in the breakdown of the dead wood fibers that make up the bulk of any tree trunk or sizable branch. The fungi typically are introduced into the living tree through insect activity or an injury that disrupts the bark, exposing the softwood cambium or the heartwood itself. Such an injury can happen when a branch breaks off in high wind or a falling branch or tree strikes the trunk. It also can happen through the action of any number of animal activities, including the foraging of insects, woodpeckers, porcupines, and beavers. Most fungi spread through the dispersal of airborne spores, which, when they land and germinate on the exposed wood, are the beginning of the fungal invasion of the tree. A fungus invades its host by literally eating its way through the wood as its mycelium grows along the wood fibers. The wood colonized by the fungus becomes punky, losing both density and structural integrity. In a standing trunk, the growth occurs more rapidly in a vertical direction than it does horizontally because the mycelial growth faces little obstacle when growing in the same direction as the wood fibers.
In a large living tree, a heart-rot fungus often is able to grow within the trunk for years without any noticeable effect, leaving a sturdy living layer of softwood cambium. The invasion becomes apparent when we see a fruiting body form on the trunk or nearby ground or when the tree is cut or falls in a storm, exposing the rotten or hollow center. A tree often continues growing for years with the fungal mycelium slowly softening and hollowing the center without visible damage or visibly slowing the growth of the tree. Hollow trunks occur only in trees living with a heart-rot fungus where, over time, the softening heartwood collapses. In most cases, this process takes years.
Other wood decay fungi begin their work following the death of a tree and start the process of softening the wood from the bark inward as they extract nutrients by breaking down the wood fibers. A dead tree, without the antifungal defenses present in living tissue, rots much more quickly than a living tree. On a large snag, there typically are several or even many different species of fungi working in concert or in different regions feeding on the tree at any moment in time. Different wood-rotting species grow better in different microhabitats created by variations in sun exposure or shade, near the ground just under the bark, or deeper in the true heartwood. Some of the better-known heart-rot fungi include the red-belted polypore, the artist’s conk, turkey tails, the various varnished conks, the tinder conk, and more fleshy fungi such as hen-of-the-woods and the sulfur shelf. A healthy forest contains scores of different species of wood rot fungi.
Deadwoodology, the study of the ecology of deadwood, is a thriving research field in which wood-decaying fungi play a major role as vigorous ecosystem engineers. The action of wood-decaying fungi increases the availability of resources such as nutrients and humus for plants and other fungi, and feeding and nesting sites for other living organisms including insects, birds, and mammals.1
Of Woodpeckers and Fungi
Most people know that woodpeckers and their relatives make their nests in cavities in the trunks of trees; fewer are aware that it is an unusually strong woodpecker that is capable of actually excavating a cavity in hard virgin living wood. Most primary cavity nesters seek dead trees or living trees whose wood has already been softened by colonizing fungi. The birds locate and make their nest holes in living trees displaying the fruiting bodies of a heart-rot fungus or ones infected with the fungus but not yet sporting a fruiting body. This is the case with quaking aspen infected with the aspen heart-rot fungus, Phellinus tremulae, which is commonly found on larger mature aspen. In a study of two sites in Wyoming, 71 percent of aspen with cavities created by sapsuckers had visible conks of P. tremulae, though less than 10 percent of all aspen in the area showed conks.2 Other studies showed similar though somewhat lower results with other bird species. Cavity-excavating birds choose trees with fungal invasion as a means of finding softer and more easily excavated sites.
One bird famous for the vigor and impact of its ecosystem engineering is the pileated woodpecker, the largest woodpecker in North America. Detritus from the feeding and nest-building activity of these birds litters the ground around the base of the trees, and the noise created by their loud excavations proclaims the presence of the otherwise shy birds. This woodpecker is referred to as a “keystone species,” one whose actions modify the forest habitat to such an extent that they single-handedly increase the diversity of species living in the environment. Most large species of woodpeckers and flickers also significantly affect the forest habitat, but the pileated makes the action of its lesser kin seem puny in comparison. The impacts of the pileated woodpecker that have earned it the keystone species status include: